1. Field of the Invention
This invention relates in general to a device for locking together inner and outer members of an oil or gas well, and particularly to a locking device for locking a drilling wear bushing within a casing hanger.
2. Description of the Prior Art
There are instances in which an inner tubular member must be releasably locked into an outer tubular member within a well. For example, in a subsea well of the type concerned herein, a subsea wellhead housing will be located on the sea floor. A casing hanger will land in the subsea housing. The casing hanger locates at the top of a string of casing that extends into the well. An annular seal will seal between the casing hanger and the wellhead housing.
If the well is to be drilled deeper, the drilling rig at the surface of the water will run drill pipe down through the casing hanger and through the casing for drilling. It is important to avoid damaging the bore of the casing hanger, because eventually, a tubing hanger may land within the casing hanger for supporting production tubing. The operator will lower a wear bushing into the bore of the casing hanger to prevent damage to the casing hanger from the rotating drill pipe.
The wear bushing needs to be retained in the bore of the casing hanger. Without some type of retention mechanism, the wear bushing might be dislodged by circulation of heavy solids or by tripping of the drill pipe through the wellhead during normal drilling operations.
There are various mechanisms for retaining wear bushings, including shear pins, lock rings, or J pins made of steel or other metallic alloys. While workable, users have experienced failure in activating or releasing these devices. It is difficult to recover the wear bushing if the locking mechanism fails to release.
Elastomeric O-rings have been used in standard and shallow cut dove tail grooves on wear bushings to prevent the buildup of fine particles between the wear bushing outer diameter and the casing hanger inner diameter. In some cases, the O-ring will expand into a groove in the wellhead housing so as to resist premature unseating of the wear bushing. The amount of retention in these cases, however, is not sufficient to serve as the primary retention means for retaining the wear bushing with the casing hanger.